A fluorine-containing polymer is prepared by emulsion polymerization method, suspension polymerization method or the like method. The fluorine-containing polymer prepared by emulsion polymerization is in a state that primary particles of the polymer having an average particle size of from several tens of nanometers to hundreds of nanometers are emulsified and dispersed in water. Since it is difficult to directly filtrate the fine primary particles in such an emulsified dispersion, those primary particles are coagulated into secondary particles having an average particle size of several microns to hundreds of microns and then filtrated.
Known methods for the above-mentioned coagulation are as follows.
(1) A method for coagulation by stirring an emulsified dispersion mechanically to apply shearing force to the dispersion and break an emulsified state.
(2) A method for coagulation by adding a coagulant and then stirring to break an emulsified state.
(3) A method for coagulation by adding a water-insoluble organic solvent to an emulsified dispersion and then stirring to coagulate fluorine-containing polymer particles in droplets of the organic solvent. After the coagulation, the organic solvent is removed (for example, JP-B-60-240713).
The both methods (1) and (2) are methods for breaking an emulsified state to coagulate polymer particles. By breaking an emulsified state of fluorine-containing polymer, coagulated secondary particles become water-repellent. The water-repellent fluorine-containing polymer particles are hardly redispersed, that is, hardly becoming in the form of slurry, which makes it difficult to feed the particles into a filtration step. Also water retained in the water-repellent coagulated particles is hardly released from the particles. Therefore it is difficult to decrease a water content greatly by the filtration method and it takes a long period of time for heating and drying. Further since a dried powder has a large space having been occupied by water, its apparent density becomes very small and handling property thereof is not so good, which results in occurrence of bridging in a powder storage tank and an increase in equipment size.
In conventional methods, primary dewatering of coagulated particles has been usually carried out by solid-liquid separation means and then heating and drying follow. Usually the solid-liquid separation of a dispersion of fluorine-containing resin is carried out by filtration, particularly by using filtration means which causes the filtrated cake to have a free surface. This is because when the fluorine-containing polymer particles are water-repellent, they are in a state of being floating on the separated water. Examples of such filtration means are Nutsche filtration, press filtration in which pressurized dispersion is fed onto the sole surface of filtration medium, and filtration under reduced pressure (suction) which utilizes the sole surface of filtration medium.
If it is intended to let a cake have a free surface, cracking occurs on the cake and water content cannot be lowered further. That phenomenon arises in case of the coagulation methods of above (1) and (2). If the water content is not lowered, in case where the obtained cake is heated and dried, a resulting powder has a small apparent density and poor handling property as mentioned above.
On the other hand, the above method (3) is a method for coagulating without breaking an emulsified state. Therefore particles do not become water-repellent and thus the above-mentioned problems in the filtration step do not arise. However since an organic solvent is used, an increase in cost is resulted and equipment for recovering the organic solvent is required and besides environmental problem arises due to exhaust water and gas.